Sustainable use of tropical fruits? Challenges and opportunities of applying the waste-to-value concept to international value chains

Agriculture and food science literature on waste-to-value applications that allow upcycling of by-product ingredients is increasing. However, this stream of research rarely takes an international trade and sustainability systems perspective. This focused review defines the term of waste-to-value and the sustainable development goals connected to it, and points to the tensions and questions arising when international trade is involved. Further, it exemplifies the challenges and opportunities of waste-to-value in tropical fruit trade through five cases of tropical fruit from South America: Green coconut, açaí, maracujá, cambuci, and jabuticaba. We present a model of the international supply chain that indicates where the opportunities of waste-to-value applications in international tropical fruit trade are situated, and discuss which future research questions need to be addressed to tackle the challenges of waste-to-value in global tropical fruit chains. Establishing the waste-to-value approach in the export of yet-underused tropical fruits can amongst others improve local employment, preserve natural resources, allow favorable use of side-streams in local energy production, environmentally friendly packaging material for transport, and add health functionalities to the end-consumer products, but challenges have to be solved in order to ensure these environmental and social benefits materialize.

Metabolic engineering of acetaldehyde production by Streptococcus thermophilus

The process of acetaldehyde formation by the yogurt bacterium Streptococcus thermophilus is described in this paper. Attention was focused on one specific reaction for acetaldehyde formation catalyzed by serine hydroxymethyltransferase (SHMT), encoded by the glyA gene. In S. thermophilus, SHMT also possesses threonine aldolase (TA) activity, the interconversion of threonine into glycine and acetaldehyde. In this work, several wild-type S. thermophilus strains were screened for acetaldehyde production in the presence and absence of L-threonine. Supplementation of the growth medium with L-threonine led to an increase in acetaldehyde production. Furthermore, acetaldehyde formation during fermentation could be correlated to the TA activity of SHMT. To study the physiological role of SHMT, a glyA mutant was constructed by gene disruption. Inactivation of glyA resulted in a severe reduction in TA activity and complete loss of acetaldehyde formation during fermentation. Subsequently, an S. thermophilus strain was constructed in which the glyA gene was cloned under the control of a strong promoter (P(LacA)). When this strain was used for fermentation, an increase in TA activity and in acetaldehyde and folic acid production was observed. These results show that, in S. thermophilus, SHMT, displaying TA activity, constitutes the main pathway for acetaldehyde formation under our experimental conditions. These findings can be used to control and improve acetaldehyde production in fermented (dairy) products with S. thermophilus as starter culture.